1. Field of the Invention
The present invention relates to a valve spring device incorporated in an engine of, for example, an automobile and a valve train using the valve spring device.
2. Description of the Related Art
A valve train of an internal combustion engine (hereinafter referred to simply as the engine) includes an intake valve, an exhaust valve, a valve spring, a drive mechanism, etc. The valves serve individually to open and close ports in the cylinder head. The valve spring urges these valves in their closing direction. The drive mechanism is provided with a cam that drives the valves in their opening direction. A mounting load and a maximum load of the valve spring are set to appropriate values such that the valves do not undergo a jump or bounce when the engine is turning fast. The jump is a phenomenon that a valve that is opened or closed at high speed by a cam hops up above a predetermined open position near the maximum lift of the valve. The bounce is a phenomenon wherein the valve is repelled by a valve seat and fails to maintain a fully closed state when it is closed as the cam rotates.
It is generally known that the valve spring causes surge if it is driven at high speed when the engine is turning fast. If the surge occurs, the load of the valve spring fluctuates. In particular, a reduction of the load that is caused when the valve spring is deformed by the surge in a direction to compress itself brings about the jump or bounce. Thus, the maximum load of the valve spring is set to a relatively high value in consideration of the surge. The higher the load of the valve spring, however, the greater a friction loss of a valve train system is, so that the fuel efficiency of the engine may be worsened. Thus, the load of the valve spring is expected to be reduced.
On the other hand, a double-pitch coil spring has been proposed as a means to prevent an increase in surge. The double-pitch coil spring includes a small-pitch portion with smaller wire turn pitch and a large-pitch portion with larger pitch. If surge occurs, turns of the small-pitch portion repeat contact and separation, thereby suppressing the surge to a certain degree.
In this double-pitch coil spring, however, both the small-pitch and large-pitch portions vibrate to reduce the load, depending on the resonance frequency, when the surge increases. In some cases, therefore, the jump or bounce cannot be suppressed. In the double-pitch coil spring, moreover, a twisting vibration caused by the surge propagates from the large-pitch portion to the small-pitch portion. Thus, the double-pitch coil spring has a problem that its surge suppressing effect is not sufficiently high for a large surge, in particular.
A valve spring device for surge prevention is described in Jpn. Pat. Appln. KOKAI Publication No. 2000-240705. In this valve spring device, a valve spring, formed of a compression coil spring, and an elastically deformable washer are arranged in series with each other. A surge wave that is produced in the valve spring is elastically absorbed by the washer.